Conventionally, a motor vehicle has to be capable of emitting, selectively, a so-called dipped beam which is generally defined by a cut-off line so as to avoid dazzling the drivers of oncoming vehicles, and a so-called main beam for illuminating the road well ahead of the driver of the vehicle. These functions are provided by means of either one or two pairs of headlights.
The headlights are generally illuminated at night and extinguished in daylight. However, there is today a tendency to regard it as being desirable to have the headlights lit in daylight, not in order to illuminate the driver's field of vision, but rather in order to provide a strong signaling or indicating function, especially for warning pedestrians, but without any inconvenience or detriment to the observer.
This function is commonly referred to as that of a "day running light" or DRL, and is known at the present time in the context of headlights with filament lamps. In order to give the required reduced light intensity to headlight driving beams for use in daylight, such as to make them capable of fulfilling this latter function, it is also known to energise the filament lamps of such headlights at a lower voltage, which is typically only a few volts, than their nominal supply voltage which is usually about 13.5 volts.
In addition, when a headlight is used in daylight driving in its dipped mode, which may involve a reduction in the supply voltage of its filament lamp, then this daylight use becomes added to its night time use. The light is used in the dipped mode far more than in its main beam mode, having regard in particular to the substantial worldwide increase in road traffic. As a result, the lamps (or the filaments in the case of twin function lamps) which provide the dipped beam function will fail much sooner than those lamps or filaments which are dedicated to the main beam function, having regard to their useful life which is, by nature, limited.
In addition, in the case where dipped beam headlights of the elliptical type are used in daylight driving, these headlights being of a kind which is well known as emitting an extremely limited quantity of light above the cut-off line, it is found that the required daylight signaling function is not at all satisfactory. In particular, in full sunlight, an observer who is a few meters or tens of meters in front of the vehicle is in danger of not even being able to see that the headlights are lit at all.
In parallel with the foregoing, we see today development of headlights which no longer have filament lamps, but which are instead equipped with discharge lamps, the latter being well known for their excellent light output for a given consumption of electrical power.
However, it is not possible to operate these headlights in the DRL mode. In this connection, if the supply voltage to the discharge lamp is reduced, then firstly the arc will tend to be subject to unexpected and badly controlled extinction, and secondly the useful life of the lamps is considerably reduced. Moreover, if in order to overcome this limitation it is decided to make use of headlights with discharge lamps with their normal light intensity, it is then found that dipped-beam lights give very imperfect signaling. The latter is too strong below the cut-off line but too weak above it, while at the same time the main beam headlights provide generally excessive intensity which is strongly dazzling even in full daylight.